This invention relates to the precision coating of surfaces and more particularly to extrusion coating of substrates using a configuration wherein a coating head moves across a substrate.
It is often necessary or desired to provide a precision coating of a particular substrate such as a glass panel. For example, in the video electronics industry it is often desired to coat panels which will serve as flat panel displays (FPD) to be incorporated into television sets, computer monitors and the like. It is important in such applications to ensure the accuracy and consistency of coating thicknesses across the panel.
A commonly employed method of coating flat panel displays is to have a stationary head extruding fluid at a particular rate over linearly moving panels. Using such a configuration, the coating consistency is dependent upon a number of parameters such as the gap between the head and the panel surface, the variation in this gap as the panel moves, the dimensional consistency of the panel, the mechanical tolerances of the extrusion orifice or slot, the pump characteristics, and the presence of gas or air bubbles in the coating material. Additional factors affecting variation in the thickness in the coating across the area of the panel will be the consistency of fluid flow rate through the extrusion head, and the consistency of linear velocity of the panel under the head as well as the ability to maintain steady movement, as measured in each of the x, y and z planes, of an often large substrate. The above all represent problems in the art.
In the context of this discussion, the length of the dispensing head refers to the span of the head, generally in a direction perpendicular to the coating direction. This xe2x80x9clengthxe2x80x9d of the dispensing head may correspond to the same direction as the width of the substrate to be coated, since the dimension of the substrate concerned may in fact be the shorter of the two horizontal dimensions of said substrate. In addition to the considerations of distance between various key elements is the issue of vertical flexing in a extrusion head across its own length. The extent of this problem will depend upon the nature of the support structure for the head as well as the length and density of the head structure. To the extent that such vertical flexing is present, it presents the problem of variation in height between the head and the panel along the length of the head.
The moving panel approach requires a large footprint for the overall mechanism because there must be at least enough space set aside for the full area of the panel on both sides of the fluid extrusion means. There is also a need for leveling the panel throughout its travel underneath the extrusion means. Further, the disadvantages of a large footprint requirement and leveling issues increase as the size of the panel increases. Therefore, it is problem in the art that the system footprint must be at least double the area of the panel to be coated. It is also a problem in the art that there could be variation in height between the head and the panel along the length of the panel.
In order to avoid dripping or smearing coating material which has gathered around the extrusion head after a coating operation, it is often necessary to clean the extrusion head before a new coating operation begins. In the prior art, cleaning of extrusion mechanisms is usually accomplished manually, potentially leading to inconsistent results and disruption and delay of the coating operations. Therefore, it is a problem in the art that manual cleaning operations are inconsistent and unreliable.
In order to ensure that coating material is applied consistently and evenly right from the start of the coating operation, it is desirable to ensure that a bead is fully and properly formed at the extrusion head prior to starting the coating process. A problem in the prior art exists with respect to properly priming fluid extrusion heads so as to ensure that a proper bead is formed prior to extruding fluid over the panel, and that a consistent rate of coating fluid flow is thereafter achieved as the full area below the extrusion head must be maintained open for the passing of a substrate thereunder, thus making it difficult to provide any priming mechanism.
Generally, in prior art coating systems, there is a single pump mechanism located remotely from the extrusion head with appropriate fluid conducting means leading from the pump the head. The use of a single pump, while perhaps economical, makes it difficult to precisely control fluid flow at the extrusion head. Specifically, it may be difficult to start and stop at precisely defined moments and to establish the precise fluid flow rate desired. Some prior art systems have used two pumps.
In the prior art, the fluid delivery means, including fluid supply, pumps, and fluid extrusion head assembly were all part of a single integrated coating apparatus assembly. As such, when it was necessary to change coating fluids, or perform other operations on the fluid delivery means, the entire coating apparatus would be idled. Fluid changeover operations include time consuming tasks such as cleaning all tubing, pumping mechanisms, and essentially all surfaces where residue of the previous coating material could be present. This thoroughness is necessary because of potentially dangerous chemical reactions between two different coating materials to be used in succession, and the possibility of cross-contamination between materials used in different processes. The idle time for the coating apparatus is expensive and wasteful given that mechanisms unrelated to the fluid delivery system are idled by the operations necessary for fluid changeover. Accordingly, a need exists in the art for a system and method wherein a chuck assembly adapted to position and hold substrates to be coated as well as other components and materials used in the coating process, but not part of the fluid delivery system are not left idle during fluid delivery system cleaning operations.
In prior art systems, variation in the height of the extrusion head with respect to the panel can cause breaking of the coating bead and variation in coating thickness. The causes of such height variation include part dimension variation, part placement error, and gradual drift in machine dimensions over time. Accordingly, there is a need in the art for a system and method for ensuring constant extrusion head height over the panel being coated.
Accordingly, a need exists in the art for a system and method for providing a uniform coating of a desired thickness on a relatively large substrate, including panels of various shapes and sizes, while providing efficient use of a coating material.
A still further need exists in the art for a system and method for coating substrates which will minimize the footprint of the coating system.
A still further need exists in the art for a system which is adaptable to very large substrate sizes.
A still further need exists in the art for a system in which a constant extrusion head gap is maintained irrespectively of flex associated with the use of a linear extrusion head.
A still further need in the art exists for a cleaning station whose functions are easily accessible to a fluid dispenser such as an extrusion head at appropriate times such as between coating operations.
A still further need in the art exists for a priming station which can be accessed easily by a fluid dispenser such as an extrusion head at appropriate times such as between coating operations.
A still further need in the art exists for more precisely controllable flow of coating material at a fluid dispenser such as an extrusion head.
These and other objects, features and technical advantages are achieved by a system and method which utilizes extrusion or other controlled delivery process to precisely place a coating material on a substrate. Preferably, the coating delivery system includes a, preferably stationary, substrate holding or positioning mechanism and a shuttle mechanism carrying a fluid delivery device across the entire length of the substrate while spanning the width of the substrate.
In the context of this discussion, the length of the dispensing head refers to the span of the head, generally in a direction perpendicular to the coating direction. This xe2x80x9clengthxe2x80x9d of the dispensing head may correspond to the same direction as the width of the substrate to be coated, since the dimension of the substrate concerned (the one parallel to the span of the dispenser) may in fact be the shorter of the two horizontal dimensions of said substrate. This explanation is offered to avoid any possible confusion arising from use of the terms xe2x80x9cwidthxe2x80x9d and xe2x80x9clengthxe2x80x9d in the following and is not intended to limit the scope of the invention.
The preferred embodiment of the substrate positioning mechanism utilizes a chuck which holds a substrate in place adapted to allow a shuttle mechanism transporting a fluid dispenser to traverse the full length of the substrate to be coated. It is easily recognized that a lower bound on the footprint of a substrate coating system according to the present invention is the area of the substrate itself. Using the configuration of the present invention, the footprint of the apparatus in the horizontal plane is much reduced with respect to a configuration in which the substrate travels a distance equal to its own length underneath a fluid dispenser. In the present configuration, the length of the system need only exceed the length of the substrate by the amount necessary for the fluid dispensing mechanism to move clear of the substrate, for purposes of substrate placement and removal, and possibly for the placement of utilities to service the fluid dispenser in between coating operations.
The configuration of the present invention is adaptable to large substrate sizes as the nature of the chuck assembly design would change little with increasing substrate size. A single coating apparatus can be used with substrates of different sizes by employing a head of appropriate length and ensuring that the shuttle mechanism has sufficient travel to cover the lengths of the various substrates to be coated.
Where a larger substrate cannot be accommodated by a particular coating apparatus, the principal changes required for such apparatus required for a larger substrate size would be to appropriately increase either the width and/or travel of the shuttle mechanism and length (or xe2x80x9cspanxe2x80x9d) of the fluid dispenser, and to adjust the size the chuck. Alternatively, where the increased size of the substrate results from an increased length, the present invention may be adapted to provide sufficient movement of the chuck to allow the combination of moving head and moving chuck to fully coat the substrate without significantly increasing the footprint of the coating system. Accordingly, in an alternative embodiment, a substrate chuck movable between first and second positions moves the substrate to cooperate with the above described mount of the fluid delivery head to provide a uniform coat of fluid to the substrate.
A shuttle mechanism which carries the fluid dispenser preferably rides on an air bearing or alternative precision support and guidance mechanisms such as rolling contact with a rail system, or low friction contact surface, located underneath the chuck assembly, the shuttle mechanism thereby forming a single continuous rigid loop structure when a fluid delivery apparatus such as an extrusion head is engaged therein. The rigidity of this design optimizes the precision with which the coating apparatus can operate. This configuration also minimizes the width of the apparatus by obviating the need for a support surface beyond the width of the chuck assembly, thereby further reducing the footprint of the coating apparatus. However, the shuttle mechanism, with its air bearing below the chuck, a carriage to carry the fluid dispenser above the chuck and substrate, and structural links connecting the two, effectively envelops the chuck thereby restricting the permitted thickness of the chuck assembly and equipment contained therein. In an alternative embodiment, the air bearing or other support and guidance mechanisms may be located to the side of the chuck assembly.
A component of the preferred embodiment chuck is a lift plate mechanism which lowers and raises the substrate within the chuck for the purposes, respectively, of loading of substrates onto the chuck, and removing substrates from the chuck. The above described constraint on the vertical dimension of the chuck forces the lift plate mechanism to accomplish the required vertical displacement of the substrate while minimizing the height of the mechanism. This is accomplished in a preferred embodiment by using motion in a direction not so tightly constrained such as by horizontally displacing diagonal wedges toward rollers attached to vertically oriented lift pins, thereby causing the pins to displace vertically as the wedges move horizontally. Once loaded into the chuck, the substrate is preferably held in place by a standard vacuum mechanism, or by alternative mechanisms including but not limited to clips, clamps, or detents. The horizontal displacement of the wedges toward the roller-based pins can be accomplished by a number of means including relay activated air cylinders, electromagnetic coils, electric motors, or by hydraulic action.
When using the shuttle arrangement described above, wherein the fluid dispenser is supported thereby such as at both ends, the fluid dispenser may flex vertically at any given point between the points of support by an amount roughly proportional to the distance of such point from the nearest support.
Accordingly, the preferred embodiment chuck is adapted to hold the substrate with a corresponding amount of flex. A chuck holder is preferably used to provide the chuck, and therefore the substrate, with the desired amount of flex. In a preferred embodiment, the chuck holder comprises a frame, structure, preferably including a provision for adjusting the dimensions of the chuck holder, suspended above the shuttle mechanism transport surface, and attached to the coating apparatus, at a plurality of points preferably just outside the range of travel of the shuttle mechanism so as to minimize the system footprint. The chuck holder preferably further comprises a plurality of chuck supports, preferably movable along the structure, which will interface with the chuck when the chuck is placed on the chuck holder.
The geometry of the suspended frame structure and the location of the chuck supports are such as to support the chuck at its edges and preferably not under its center. This arrangement is designed to permit the chuck and any substrate placed upon it to flex vertically along the axis perpendicular to shuttle mechanism travel. This vertical flexing is intended to match the vertical flexing along this same axis expected in the dispensing head.
In a preferred embodiment of the present invention, utilities for servicing the fluid dispensing head may be located within the range of travel of the fluid dispensing head as carried by the shuttle mechanism. With such an arrangement, the shuttle can be automatically programmed to stop at these utilities in between coating operations or at other appropriate times. A set of utilities could include a scrubbing station at which bulk coating material would be removed from the dispensing head through a combination of physical scrubbing with brushes in combination with use of a solvent.
Another operation among these utilities could consist of a rinsing station at which a powerful solvent removes any material remaining from the most recent coating operation, even if the dispenser has been cleaned at the scrubbing station. Yet another operation among these utilities could consist of a priming station at which the dispensing head could be placed so as to ensure that a full and consistent bead of coating fluid is made ready at the dispensing head in preparation for the next coating operation. A preferred embodiment for such a priming station consists of rotating cylinder upon which coating fluid is placed in the smallest quantity necessary to establish a consistent bead. In this embodiment, holding the dispensing head stationary in proximity to the rotating cylinder effectively simulates moving the dispensing head over a certain length of surface material.
In a preferred embodiment of the present invention, a primary pump located remotely from the dispensing head would pressurize the fluid connections leading up to a dispensing head assembly, and a second smaller pump, integrated into the dispensing head assembly, is able to precisely control the dispensing of fluid from the dispensing head. In an alternative embodiment, a single pump can be used to perform all required fluid pumping functions within the apparatus.
In a preferred embodiment of the present invention, the fluid supply, pumping means, fluid dispensing head and utility stations would all be located on a cart removably attached to a main operating station containing the chuck and shuttle mechanism. Upon attaching the cart to the operating station, the shuttle mechanism would be attached either manually or automatically to the dispensing head which initially resides on the cart. The shuttle mechanism then, preferably under computer control, is able to move the dispensing head to the previously discussed utility stations, over the full length of the substrate to be coated, and when ready, back to the appropriate place on the removable cart.
With such an arrangement, each cart may be associated with a particular fluid or with a particular size or type of head. When a cart becomes unusable either because the fluid supply has been exhausted, becomes unusable due to degradation over time, or because the current manufacturing process requires using a different coating fluid, the used cart can be readily and rapidly disconnected from the main operating station. A new cart can then be immediately attached to the main station, and the shuttle mechanism again attached to the fluid dispensing head on the new cart. Coating operations can thus quickly resume without waiting for the time consuming task of cleaning and readying for operation the fluid system on the old cart. With this embodiment, the old cart can be cleaned and prepared for renewed operation in parallel with the resumption of coating operations at the very same main operating station. The idle time experienced in the systems of the prior art is thereby avoided.
In a preferred embodiment of the present invention, there is provision for real time sensing and adjustment of the height of the dispensing head with respect to the substrate being coated. Maintaining a constant height is critical to maintaining a good bead, and providing a continuous and consistent coating across the entire substrate. Variation in the height of the dispensing head over the substrate can result from variation in physical dimensions of the substrate, warpage of the substrate, or part positioning error.
An independent contributor to possible variation in the height of the dispensing head with respect to the substrate stems in fact from variation of the height of the shuttle mechanism mounting platform with respect to the chuck, that is to say a variation in machine dimensions rather than just variations in part placement and part dimensions. This variation in machine dimensions can result from a slow drift in mechanical dimensions over time, such as from the gradual bending of metal parts, wearing of certain surfaces, and thermal effects.
Height variation from either or both of the above sources can be addressed by employing a height sensor feeding information to a control system which activates a motor to drive the dispensing head higher or lower as the sensing data dictates. The height sensor is taught an appropriate xe2x80x9czeroxe2x80x9d point representing the correct height of the dispensing head, and any subsequent deviation from that point results in an error signal causing the control system and motor to correct the dispensing head""s height. Preferably, the rate of adjustment in the height of dispensing head is tempered so as to ensure that an extrusion bead will not be broken. Sensing methods available for this purpose include but are not limited to mechanical contact sensing, preferably with roller contact, optical, air cushion, and ultrasonic.
In a preferred embodiment of the present invention, variation in the planar consistency of the substrate is compensated for through deployment of a micro deforming chuck. Preferably, the micro deforming chuck is composed of a rigid lower layer, semi-rigid upper layer, and a middle layer composed of piezoelectric crystals or other micro-deforming means. Raising and lowering the voltage applied to the piezoelectric middle layer permits this middle layer to be raised and lowered at strategically selected points so as to make the height of the upper level of the substrate uniform across the substrate. Further flexibility can be added by enabling three axes of motion to the chuck to provide for greater adjustment of surface levels than possible with micro-deformation of the chuck alone.
In another preferred embodiment of the present invention, fluid exits from a dispensing head only along selected portions of its length, thereby enabling segment coating. Segment coating is the ability to form multiple devices adjacent to each other on a substrate so as to obtain a matrix of devices which can be separated after liquid deposition. Unlike other coating techniques such as spin coating, extrusion coating is better suited to perform segmented coating since it directly deposits precise layers of subject fluid. At this time, there have been no successful attempts at segment coaters in the industry. The ability to segment coat a substrate is a critical step in the technology since it can reduce the number of runs by producing two segments at once, and it can make the use of larger areas of substrate more efficient.
In particular, there have been no adequate systems for handling large area substrates which will supply multiple displays or devices. In addition to the difficulties of obtaining an even surface, the throughput time of the coating equipment is very important. Throughput of a coating module is determined substantially by the length of the coated area divided by the linear rate of coating. Obtaining a throughput time that effectively allows for coating of a large area substrate has not been accomplished in the prior art.
There remains a long-felt need in the industry to more efficiently dispense subject fluid onto a substrate so as to form multiple devices on a single substrate. Segmenting the deposition of coating fluid in this manner permits separate coating fluid streams to be deposited onto separate substrates or separate portions of a common substrate without interfering with one another during deposition. One approach to segmenting fluid deposition involves placing a die lip over the dispensing head orifice. Each die lip is a separate part which is removably attachable to the dispensing head. The die lips of fluid delivery extrusion heads may have extrusion orifices of varying lengths to accommodate the substrates and/or fluids which are to be processed. Such an arrangement can permit a variety of different substrates of potentially varying dimensions to be processed in one sweep of the dispensing head thereby optimizing the production efficiency. Alternatively, segment coating can be achieved by using dispensing heads which may be extrusion heads with a plurality of extrusion orifices in them to permit fluid to exit from a plurality of slots rather than along the full length of the dispenser.
A further inventive mechanism comprises a control system combined with multiple extrusion head and pump mechanisms for applying a uniform and segmented layer of liquid to a substrate, preferably a large area substrate. In its preferred embodiment, the extrusion heads include a liquid-containing chamber and dispensing slots in communication with the chamber. A pump, integrally mounted to the extrusion head itself, provides a steady-state fluid flow of liquid to the slots on the extrusion head. Valves between the slots and the chamber control the dispensing flow so as to allow for differences in adjacent segments. The integrally mounted pumping means enables precision control of flow conditions within the head in a manner that avoids transient perturbations during initial extrusion startup. Fluid is supplied to the pump from a fluid supply bay remotely located from the pump. The fluid supply bay includes a supply pump, a fluid reservoir and means for filtering the fluid.
In a preferred embodiment the control system of the coating apparatus of the present invention consists of an adaptive type control unit, such as a neural network system. For example, there may be a pressure sensor within each head manifold and a vision sensor on the substrate chuck as well as a vision sensor for each of the bead formers on the extrusion head, preferably CCD cameras. The process control system can also be extended to monitor other attributes such as the steady state flow from the pumping means to the extrusion head.
Accordingly, the control system may analyze and/or store empirical data in order to adaptively operate the systems of the coating apparatus to provide a desired coating on a substrate.
Accordingly, it is a technical advantage of the present invention that the moving head configuration minimizes the footprint of the coating apparatus.
It is a further technical advantage of the present invention that the coating apparatus is adaptable to very large substrate sizes, including for example, 1200 mm by 1600 mm.
It is a still further technical advantage of the present invention that the substrate to be coated will flex vertically in conjunction with vertical flex in the coating head so as to minimize the variation in distance between the head and the substrate along the length of the coating head or dispensing head.
It is a still further technical advantage of the present invention that the substrate to be coated can be accepted into, and presented for removal from, the coating system while minimizing the thickness of the chuck holding the substrate.
It is a still further technical advantage of the present invention that a plurality of means is provided with which to accomplish segment coating which can be applicable to coating a plurality of substrates at once.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.